The present invention relates to the art of connecting tubing, pipes, and conduit. It finds particular application in conjunction with the interconnection of large diameter, smooth interior wall corrugated plastic tubing and will be described with particular reference thereto. However, it is to be appreciated, that the invention will also find application in conjunction with smooth-walled, corrugated, and other types of pipe and tubing.
Large diameter plastic pipe, e.g. 24 inch (60 cm) plastic pipe, is commonly shipped and handled in sections, e.g. 20 foot (6.5 m) sections. Each length has a stiff, self-supporting outer wall which includes alternating peaks and valleys that define a series of corrugations. Optionally, a lighter-weight continuous tubular liner extends along and is connected with the valleys to define a smooth interior diameter. Tubing with a smooth interior diameter has a substantially higher fluid carrying capacity than tubing with a corrugated interior of the same diameter.
To connect the lengths of tubing, external, sleeve-type couplers are commonly utilized. These couplers include a plastic sleeve or bell whose inner diameter substantially matches the outer diameter of the corrugation peaks of the tubing lengths to be connected. Typically, a corrugation or groove is defined in the center of the coupler to mark the center and to provide an abutment for the ends of the connected tubing lengths. To provide a fluid tight seal, an annular gasket is disposed in one of the corrugation valleys which will be received within the coupler. Generally, the tighter the gasket presses against the cylindrical interior surface of the coupler, the more pressure the joint will hold. To this end, the gasket is commonly defined such that it substantially fills the corrugation valley and has an outwardly cammed flap or projection. As the end of the tubing length is inserted into the coupler (or the coupler inserted around the end of the tubing length), the gasket is compressed into the valley and firmly against the interior cylindrical surface of the bell.
In the past, attempts to improve the seal and raise the failure pressure have focused on the gasket. In particular, the gaskets are now compressed sufficiently that water does not flow between the gasket and bell. To facilitate insertion with this high compression of the gasket, the interior surface of the coupler bell is commonly coated with a lubricant. This facilitates pressing the coupling and the end of the pipe together, typically with axial pressure from a backhoe. Although the gasket/bell interface withstands well over 10 psi, the couplings still fail at about 10 psi.
The inventors herein have discovered an unexpected failure mode. With reference to FIGS. 1A and 1B, the internal pressure in the pipe feeds back around the end of the pipe to the gasket. The gasket acts like a piston in the coupler sleeve. The pressure tried to push the piston or gasket out of the coupler sleeve in a direction which tries to compress the pipe axially. This first axial compressive force is combined with a second force which also urges the pipe to compress axially. The annular chambers defined between the smooth inner liner and the corrugation peaks typically have a very small weep hole to allow the pressure within these annular regions to equalize with the external environment. As the pressure in the interior of the pipe increases, there is an increasing pressure differential between the interior of the pipe and the annular regions below the corrugation peaks. This pressure differential causes the inner liner to arc into the annular regions and the corrugation peaks in the coupler bell to draw together axially or fold in an accordion-type style. Backfill around corrugations outside the coupler tend to fill the corrugation valleys and resist this accordion-type contraction. The corrugations on the high pressure side of the gasket are not subject to the first axial compressive force and are only subject to the second compressive force until the pressure equalizes through the weep hole.
As these two axial pressures cause the corrugations to contract in an accordion-like manner, the length of tubing section becomes shorter. Because the central region of the tubing section is well anchored by the backfill, the gasket and the end of the tubing move toward the central portion of the section and are withdrawn from the coupler. In a double bell connector, the end of the tubing connected to the other bell is withdrawn analogously, but in the opposite direction. This axial compression or shortening of the pipe continues until the gasket is pulled from the bell or until the pipe compresses further on one side than the other. Such uneven compression or crushing of the pipe causes a rotation of the gasket out of the vertical plane (when the longitudinal axis of the pipe is horizontal) which pulls the gasket away from the bell. The lubrication which was used to allow the gasket to seal more completely to the interior surface of the bell actually facilitates the movement of the gasket and failure of the joint. In this manner, the solution dictated by the conventional wisdom discussed above, i.e. compressing the gasket so hard that a lubricant is required for its insertion, is actually promoting the failure.
The present invention contemplates a new and improved coupling arrangement which overcomes the above-referenced problems.